Conventionally, ferroelectric memory cells are typically provided on rigid substrates, such as silicon. However, electronics become increasingly used also in non-conventional application areas and new technologies emerge for producing electronics, where use of flexible substrates is desirable or even required. This is for example the case for printed electronics, where use of a flexible substrate may be desirable or even required from manufacturing, application area, and/or cost perspectives.
Printed electronics may replace conventional electronics in case of very simple components, which can be realized less expensive by printing technologies; however, the aim is typically new application areas where conventional electronics are not suitable for technological or cost reasons. Applications for printed electronics involve, for example, tags and labels in which information can be stored. In such applications, and in principle in any electronic device, availability of memory components is crucial.
The present applicant has presented a memory technology that can be realized by printing processes, which e.g. is described in WO2006/135246. The memory is based on a ferroelectric material as the memory substance, in particular a ferroelectric polymeric material. Memory materials of this kind has proven to be rewritable and bistable over prolonged periods of time. Each memory cell is a capacitor-like structure where the memory material is located between a pair of electrodes and where the memory cell is accessed via conductors linking the electrodes to electronic driver or detection circuitry. The latter may e.g. be located on the periphery of the memory array or on a separate module. Depending on the application, memory device may contain from one individual memory cell and up to several millions of cells arranged in matrix arrays. Some basic cell architectures and array arrangements are schematically shown in FIGS. 1a-d. It may be noted that the substrate is not shown, only the electrically active part of the memory cells. Each cell can be viewed upon as a sequence, or stack, of layers arranged on the flexible substrate, the stack involving at least one electrically active part comprising two electrode layers (top and bottom) with a layer of the (insulating) memory material arranged in-between.
When fabricating ferroelectric memory cells of capacitor type it is evidently important to avoid shorts through the memory cells. Shorts are here defined by a conducting or low resistance path, compared to a desired normal situation, from one of the electrodes to the other electrode. The short circuits are detrimental to the memory cell function as they can both hide the data content of a memory cell and also deteriorate the writing of data into the memory device. The problem with shorts is typically greater when the memory material layer between the electrodes is thin. However, the thickness of the memory layer and drive voltage are typically proportional to each other, and in order to meet up with low voltage requirements, there is often no other option than using a thin memory layer. Manufacturing will always lead to some extent of memory cells being short circuited or more prone to be short circuited. It is desirable to reduce the risk of short circuits to occur.
Furthermore, printed electronic devices or components typically need to be protected against external influences, such as physical damage, but it is typically not possible, nor desirable with protection by e.g. encapsulation as in conventional electronics. Instead a desirable type of protection is an outer protective layer terminating the stack and that adds protection by e.g. providing scratch and abrasion resistance and resistance against detrimental environmental influence. A protective layer of this kind can be provided as a global layer covering multiple memory cells, e.g. by completely covering a printed memory device. Such protective layer typically need to be hard and relatively thick, such as in a range of 2-20 micrometers, and it is often suitable and desirable to use a material that can be deposited as layer in a fluid state and then hardened, e.g. by using a UV curable varnish as the protection layer.